Evaluation of the Andromas Matrix-Assisted Laser Desorption Ionization–Time of Flight Mass Spectrometry System for Identification of Aerobically Growing Gram-Positive Bacilli

Matrix-associated laser desorption ionization–time of flight mass spectrometry (MALDI-TOF MS) is a rapid and simple microbial identification method. Previous reports using the Biotyper system suggested that this technique requires a preliminary extraction step to identify Gram-positive rods (GPRs), a technical issue that may limit the routine use of this technique to identify pathogenic GPRs in the clinical setting. We tested the accuracy of the MALDI-TOF MS Andromas strategy to identify a set of 659 GPR isolates representing 16 bacterial genera and 72 species by the direct colony method. This bacterial collection included 40C. diphtheriae , 13C. pseudotuberculosis , 19C. ulcerans , and 270 otherCorynebacterium isolates, 32L. monocytogenes and 24 otherListeria isolates, 46Nocardia , 75Actinomyces , 18Actinobaculum , 11Propionibacterium acnes , 18Propionibacterium avidum , 30Lactobacillus , 21Bacillus , 2Rhodococcus equi , 2Erysipelothrix rhusiopathiae , and 38 other GPR isolates, all identified by reference techniques. Totals of 98.5% and 1.2% of non-Listeria GPR isolates were identified to the species or genus level, respectively. Except forL. grayi isolates that were identified to the species level, all otherListeria isolates were identified to the genus level because of highly similar spectra. These data demonstrate that rapid identification of pathogenic GPRs can be obtained without an extraction step by MALDI-TOF mass spectrometry.